The present invention relates to a mass spectrometer and method of mass spectrometry.
Orthogonal acceleration Time of Flight mass analysers are known wherein ions having approximately the same energy are passed through a space in which an orthogonal acceleration field is periodically applied. The length of the orthogonal acceleration region, the energy of the ions and the frequency of the application of the orthogonal acceleration field determine the sampling duty cycle for sampling ions for analysis in the Time of Flight mass analyser. Ions having approximately the same energy but having different mass to charge ratios will have different velocities and hence will have different sampling duty cycles.
The maximum ion sampling duty cycle for an orthogonal acceleration Time of Flight mass analyser of conventional design when used with a continuous ion beam is typically about 20-25%. This is only achieved for ions having a maximum mass to charge ratio and the ion sampling duty cycle is less for ions having lower mass to charge ratios. If ions having the maximum mass to charge ratio have an mass to charge ratio of mo and the sampling duty cycle for these ions is Dco, then the sampling duty cycle Dc for ions having a mass to charge ratio m is given by:
                    Dc        =                  Dco          ·                                    m              mo                                                          (        1        )            
It can be shown that the average sampling duty cycle is equal to ⅔ of the maximum sampling duty cycle Dco. Hence, if the maximum sampling duty cycle is 22.5% then the average sampling duty cycle is 15%.
If ions are stored in an ion trap upstream of an orthogonal acceleration Time of Flight mass analyser and are then released into the mass analyser as a series of packets, rather than allowed to flow continuously, then the energisation of the pusher electrode can be synchronised with respect to the release of each packet of ions. However, ions having the same energy but different mass to charge ratios will enter the mass analyser with different velocities. Hence, ions with different mass to charge ratios will arrive at the orthogonal acceleration region adjacent the pusher electrode at different times. The time delay between the release of ions and the subsequent energisation of the pusher electrode determines the mass to charge ratios of the ions that are orthogonally accelerated and which are therefore transmitted into the orthogonal acceleration drift region of the mass analyser. For these ions the duty cycle can now be increased to substantially 100%. However, ions having other mass to charge ratios will not be arranged so as to be adjacent the pusher electrode at the time when the pusher electrode is energised and hence these ions will have lower sampling efficiencies. Ions with very different mass to charge ratios will have a sampling efficiency of zero.
An alternative approach is to trap and store ions in a mass selective ion trap such as a 3D quadrupole or Paul ion trap. Such ion traps may be operated so as to permit only ions having a selected mass to charge ratio or a range of mass to charge ratios to be ejected from the mass selective ion trap. Accordingly, ions having a relatively narrow range of mass to charge ratios can be arranged to be ejected from the ion trap. The time delay between the ejection of a packet of ions from the ion trap to the energisation of the pusher electrode can be set to be that required for the range of mass to charge ratios of the ions released from the ion trap. Ions having other mass to charge ratios are retained within the ion trap and can be released in a subsequent packet of ions released from the ion trap. For each cycle, ions having a different range of mass to charge ratios can be released from the ion trap and the delay time can be set as appropriate for that range of mass to charge ratios. Eventually all the ions within the ion trap may be released and mass analysed.
Quadrupole ion traps may be scanned to mass selectively eject ions in two distinct ways. Firstly, either the RF voltage and/or the DC voltage may be scanned to sequentially move ions from within regimes of stable ion motion to regimes of unstable ion motion. This is known as mass-selective instability. Secondly, an ancillary AC voltage (or tickle voltage) may be applied to the end caps of the quadrupole ion trap to resonantly excite and eventually eject ions having a specific mass to charge ratio value. This is known as resonance ejection. The RF voltage or the frequency of the AC tickle voltage may be scanned to sequentially eject ions having different mass to charge ratios.
It may be desired to scan down in mass to charge ratio very quickly. To release ions in the axial direction in reverse order using mass-selective instability it is necessary to scan such that ions sequentially cross the βz=0 boundary of the stability regime. This can be achieved by progressively applying a reverse DC voltage between the centre ring and the end caps of the ion trap or by scanning both the DC voltage and RF voltage.
Another method of ejecting ions in reverse order of mass to charge ratio is to apply a small DC dipole between the end caps of the ion trap. Ions with the smallest βz values are displaced towards the negative cap. As this voltage is increased first ions having relatively high mass to charge ratios and then subsequently ions having relatively low mass to charge ratios are ejected. This has the advantage of ejecting ions in one axial direction only. The method of resonance ejection may also be used to eject ions in reverse order of mass to charge ratio.
The known arrangements described above suffer from the fact that ions will be resonantly ejected from the quadrupole ion trap with relatively high energies and with a relatively high spread of energies. The energies of the ions and the energy spread may be many tens of electron volts or even hundreds of electron volts depending upon the precise method of scanning. Furthermore, the ion energies and energy spreads will vary with mass to charge ratio depending on the method of scanning.
It will be appreciated that since it is desirable that all the ions arrive at the orthogonal acceleration region with approximately the same energy then the known approach may be problematic.
It is also desirable for an ion beam to be tightly collimated as it passes through the orthogonal acceleration region of a orthogonal acceleration Time of Flight analyser in order to achieve good mass resolution. Since ions ejected from a quadrupole ion trap will have relatively large energies and relatively large energy spreads then conventionally it is usually necessary to reject a considerable proportion of these ions in order to obtain a narrowly collimated ion beam. This in turn reduces sensitivity.
It is therefore desired to provide an improved mass spectrometer.